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Little background....Its a 00 5 spd forester, it has a weird powerband....like 3500-3700 area feels like it hits a powerband....almost like it wasnt firing perfect then it goes....Does it most of the time...

It isnt in the engine as i have also changed out the engine, spark plugs, coils, wires and pretty much everthing....Still does it. It has had a p0420 for cat eff. since i also have a UEL Header...But does it as well after i reset ecu or erase the code.....

Kinda weird but its diff feelable and kinda annoying....anyone else expeirence this or could this be ecu related.

A bad front O2 sensor can royaly screw closed loop fueling and make CL performance suck but once it transitions to Open Loop, BAM torque and power. If the high range trims are off (from bad O2), even high rpm/load ranges can suffer. Chking fuel trims with an OBD scan tool should be able to rule out O2 sensor issues.

A bad front O2 sensor can royaly screw closed loop fueling and make CL performance suck but once it transitions to Open Loop, BAM torque and power. If the high range trims are off (from bad O2), even high rpm/load ranges can suffer. Chking fuel trims with an OBD scan tool should be able to rule out O2 sensor issues.

This is just another possibility

About that checking fuel trims thing. . .What should I/we be looking for? I've been thinking my O2 sensor has been bad for a while, but in the last week or so I've been feeling a surging around 3-3.4K, and I'm thinking that may be at the OL/CL transition. . .(Phase 2 EJ22, MY00)

About that checking fuel trims thing. . .What should I/we be looking for? I've been thinking my O2 sensor has been bad for a while, but in the last week or so I've been feeling a surging around 3-3.4K, and I'm thinking that may be at the OL/CL transition. . .(Phase 2 EJ22, MY00)

I'm not sure, but I think anything before 02 will not work with a tactrix or equivalent cable and romraider software on laptop, so you will need an obd scanner. You might be able to talk the guys at your local autozone store into looking at your furl trims with their scanner. they pull trouble codes for free and this should take them the same amount of time.

There is typically 4 long term fuel trim ranges usually designated "A" to "D". A being idle, B being low load, C being cruise area and D being above cruise/high load. If any, especially the C and D are more than a cpl points away from zero then the O2 could be the reason. An exhaust leak before the O2 or an intake leak after the MAF and some less common things can cause weird trims, too.

I'm not very familiar with pre 05 subarus, so if this does not apply to yours I apologize. But I think most obd based EFI systems work on a similar program design.

You are correct about the CL/OL transition being around 3.5-4k rpm in most gears

Thanks for the ideas. FWIW, I can watch my trims in (near) real time with my Actron scanner. . .so this might be a decent option for others who can't use the Tactrix scanner. I'm just curious as to what I want to be looking for. Either way, at 116K I'm probably overdue for the sensor anyway. . .New sensor is in the mail, hopefully the weather will clear and I'll be able to get under the car this week

I havea feeling that the trims you see in real time are short term and they won't be as useful unless your sensor is getting worse while you are looking at them or if you have just rest your ecu. If you can find the long term trims, anything more than 5% is far enough off that something is up IMO. That's as long as everything intake wise is stock and in good working order.

My long-term fuel trim was at -3.4%. New O2 sensor went in today, no issues, no CEL . After an ECU reset (pulled off the negative), how long should I expect it to take to learn the new one, and see if it was money well spent?
As far as the knock sensor---when mine went you could actually feel the car pull timing (like getting gut-punched) as the CEL went on. Seemed like it made way more of a difference at higher RPM, rather than lower.
Thanks for the all the help so far